Current transformer



y 5, 1932- R. H. BORKENT 7 1,865,430

CURRENT TRANSFORMER Filed June 18; 1930 a duction (pre-ma Patented July 5, 1932 UNITED STATES PATENT OFFICE nonmamm noaxniwr, or rim moon, mnnamns Application fled June 18, 1980, ierial lo. 401,978, and in the Netherlands June 25, 1929.

The invention relatesto acurrent transformer the seconda current of which can be made proportiona to'the primary current with a very high degree of accuracy. The

l invention is especially adaptable to straightthrough bushing transformers.

With orumary types of current transformers the magnetlc induction in the cores is very small therefore the magnetizing ourrent and the errors of the transformer are large. The errors canv be decreased by choosing the working range of the transformer nearertothe oint of maximum rmeability, the applicatlonof this effect ing called pre-magnetization. The invention gives a solution of the problem to decrease the error of current transformers by increasing and by adjusting the magnetic induction in the core material. The general object of the invention is to provlde parallel branchesin the secondary windingof the current transformerfgiving rise to internal currents flowing in the secondary winding. With types known in the art said increase of the magnetic 1n- 'ving the trans ormer an auxiliary winding connected to an auxiliary potential. Said auxiliary windin is'so arranged that there is neither mutua inductance between said winding and the secondary nor between the same and the primary windings of the transformer. The current in said auxiliary winding therefore does not alter the currents in the other windings of the transformer, apart from the indirect influence caused by the mcrease of the magnetic induction, which influence is exactly the purpose of the auxiliary winding. On the -contrary with types according to the invention, various parts of the secondary windings are coupled magnetically and a number of said couplings have a direct influence on the current intensities in the different parts of the secondary windin s, that is as a consequence of the mutual in uctance between some secondary coils, electromotive forces are induced in them, which electromotive forces influence the currents in the secondar, windings. Said in ternal secondary currents together with the primary current produce the ampere turns,

etization) is obtained by necessary to obtain the suitable value of the magnetic induction in the various parts of the core material. 7

According to the invention the secondary winding possesses a number of coils, which as are connected electrically as well as coupled magnetically in such a manner that a number (that is more than one) of said magnetic couplings have a direct influence on the current intensities in the different parts of the to secondary winding.

In this specification the total of succeeding turns embracing the same magnetic flux and flown through by the same current, is

called a coil. As a limit case such a coil may OI consist of one sin le turn.

Accordin to a c aracteristic of the invention, the suitable values of the internal current intensities in the different parts of the secondary windings are adjusted by 'ving 1a the secondary windings parallel branc es'in such a manner that in more than one of the branches two or more coils, diflering in their number of turns, and placed on more than one or on separate cores, are connected in 15 serles.

An impedance may be connected in parallel to a coil; or one or more bridge branches, wherein an electromotive force is induced, may be inserted% e proper values of said currents may also be obtained or adjusted by two or more secondary circuits, which are separated electrically. In constructional forms'embo dying the invention the distribution of said currents may still be varied by modifying the constants of the magnetic circuits. This may be brought into practice, for instance by providing magnetic shunts or air gaps in the magnetic circuits, or by the choice of special 00 sections and special core material of the different cores.

The invention will be better understood from the following description taken in connection with the accompanying drawing, in I which Fig. 1 is a diagrammatic view of a current transformer according to the invention, and shows an application which 'ves favourable results in a simple manner. %ig.

2 shows an arrangement almost similar to rent in t the that of Fi 1 whereb some secondary coils are partia y combine Fig. 3 is a vectorial representation of the current and volta e relations obtaining in a transformer 0 the character described in connection with Fig.-

1. In Fig. 4 a bridge branch is added to the arrangement of Fi 1, wherein an im edance is inserted. n Fig. 5 in said bridge branch also an electromotive force is inserted. Fig. 6 shows an extended arrangement of Fig. 1 with three cores. Fig. 7 shows diagrammatically how the cores may be partially combined.

Like reference characters refer to similar parts in the different figures of the drawing. In Fig. 1 the primary conductor is marked Z; the cores on which the four coils '11., 'v, w and a; are placed, are indicated by f and g. The numbers of the turns of these coils are indicated by the same reference characters. Choosing the number of the turns accordingly, it is possible to obtain in both cores magnetizing fields of suitable value, inducingelectromotive forces in said coils. The effective values of said electromotive forces are respectivel called E E,, E and E The curhe branch a, u, b, w, 0 is called L; the current in the branch at, o, d, m, c is called 1,, and the currentthrough the instrument, for instance a measuring instrument, 1,. The impedance of said three branches are called respectively 2,,, 2, (consisting of ohmic-resistance and leakage-reactances) and 2 Further I and I, are in size and phase the intensities of the primary and secondary current, these currents having a phase difference of approximately 180; W is the number'of the turns of the primary winding; L is the mean length of the lines of magnetic induction, (in the case of the cores having such a shape that magnetic circuits contain parallel branches, for instance in the case of cores of the shell-type, L may refer to the mean length of the lines of magnetic induction of the respective branch) and the real number 1" is the ratio of the current transformer, that is the ratio of the nominal values of the primary and the secondary current.

Departing from ordinary types of current transformers where the magnetizing ampere turns are equal to the vectorial sum of the ampere turns of the primary and the secondary current, that is equal to ugm ampere turns per centimeter (the deviation from this value, caused, for instance, by a compensation of the number of the secondary turns, or by a circuit shunting the secondary load, as is sometimes done in order to correct the ratio and the phase angle of the current transformer. is of no interest here,

as said complications have no influence on the order of the magnitude of the magnetomotive force) the magnetizing ampere turns of core f are equal to [,Wipuihv and those'of core 9 are equal to I, ,i' l wi-I x. Said am ere turns cause in the cores the mag netic fie ds which induce in the secondary windings the electromotive forces.E,,, E,, E. and E whereby I E w E. U E, a: and E u Refering now to the vectorial representation of the voltage relations of Fig. 3 the sum E,,,T-E., as well as the sum E,'-T-E, must be equal to the sum of the voltage between the secondary terminals of the transformer 1 .2 and the potential drop 1,, 2,, respectively 1,, a in the branch.

By way of a specific example let the cross sectional areas of the cores be equal, let 2, 0,6 ohm, z =z =L, 2 ohm, I =60 amp., I,= 0,5 amp, W,,=1, u=150, w=90, v=w=119, then the calculation of the currents and voltages according to the principles given above will show:

Eu -0,68 volt; Ih=0,16 amp.; 5 I.z.=4l,15 volt; E. =0,55 volt; Ia==0 41 amp.; E.=o,9o volt; arc id-mas a E. =1,20 volt; are (Eu-E|)=154 degrees.

Theproportion of the magnetic inductions in the cores 7' and 9 being (according to the fact that the cross sections of the cores are equal and the number of turns of the coils o and w are the same) this example shows clearly the unequal magnetization of the cores, thus departing from the already mentioned types known in the art, where the magnetic inductions in the cores are imperatively the same. An unequal undesired-magneti zation of the cores deteriorates the accuracy of the pre-magnetized current transformers known in the art. Ac? cordin to the invention the unequal magnetization is always taken into account when designing the current transformer.

It is evident that in a general way the cross sections of the cores need not be equal, the leakagereactances of the brances b and d need not be zero and the number of the turns of the coils v and a: need not be the same, as

they are in the specific example for the sake of cores may be combined, ofierin In Fig. 2 the coils-u and v, as well as w and a: are combined, offering an opportunity for savin coil material.

- In ig. 4 a bridge branch containing an adjustable. impedance 2,, is placed between thepoints band d. a T

In Fig. 5 moreoveran electromotive force is inserted in said bridge branch, which1electromotive force is obtained by induction with the aidof a third core. Said electromotive force may be regulated in size and phase, for instance by an additional winding it closed upon an adjustable impedance 2 whilst said circuit may also contain -an electromotive force. The part of the secondary winding, which is connected electrically to the secondary burden 2,, is a kind of Wheatstone bridge, the coils u, c, w and w composing-the sides of the quadrangle, and z, and the branch a being the diagonals. This permits the current I flowing through z, to be approximately independent from a, and inversely, as should be the case with a current transformer (the secondarycurrent I, should always be proportional to the primary current 1,).

. The suitable values of the currents I and L may also be obtained by using three or more cores.

Fig. 6 gives'a combination with three cores in order to influence the division of the 'current I, into 1,, and I The coils as well as the cores may be partially combined.

Fig. 7 shows diagrammatically how the an opportunity to save core material. or the sake of clearness only the windings u and w are shown. Here number of w turns embrace both cores f and g, and a number of uw turns embrace only the core f.

It fs to be understood that such changes ma be made in the arrangement, number an connection ofthe several parts as shallfall within the scope of the appended claims,

I claim: i

1. A current transformer, comprising a lurality' of magnetic cores some of them aring each a plurality of electrically con- I nected secondary coils, the several coils of at least two cores being such, that the number of turns of the coils upon the same core differ from each other, the secondary winding of the transformer comprisin parallel branches formed by coils upon a p urality of cores.

-, 2. A current transformer as s ecified in claim 1, in which in several branches several coils diifering in the number of turns and placed on separate cores are connected in series.

- 3. A current transformer as s eclfied in claim 1, in which in several branc es several coils difi'ering in the number of turns and placed'on separate cores are connected in series and including a bridge branch, in which electromotive forces may be induced.

4. A current transformer as specified in claim 1, in which in several branches several coils differing in the number of turns and placed on separate cores are connected in series and including a bridge branch, in

which electromotive forces may be induced,

and a regulating impedance added to the secondary winding.

ROELOF HERMAN BORKENT. 

